1. Technical Field
This invention relates to the field of testing transducers for transformation of mechanical energy to electrical energy such as accelerometers and velocity transducers, and more particularly to an apparatus and method for testing accelerometers and velocity transducers at low vibration frequencies.
2. Description of the Relevant Art
Accelerometers and velocity transducers are used generally to sense and provide an electrical output signal which may be utilized to measure aircraft frame vibration, and in particular to measure helicopter frame vibration that occurs when the main rotor blades of the helicopter are unbalanced and/or out of track. The accelerometer and the velocity transducer provide an electrical signal proportional to mechanical force exerted thereon at its location in the helicopter motive system.
Typically, accelerometers and velocity transducers are tested by exciting the transducer with an electrodynamic shaker that provides a sinusoidal excitation of the transducer at or near a reference frequency of 1000 Hertz, simultaneously measuring the response of the transducer to the reference excitations, and then comparing the measured response with predetermined limits to ascertain whether the transducer is functional. Optimally, the excitation of the transducer is perfectly sinusoidal so as not to introduce any harmonic excitations to the transducer under test. The electrodynamic shaker commonly includes a coil wrapped around an armature and produces sinusoidal excitation of the armature by varying the electromagnetic field surrounding the coil.
However, the performance of electrodynamic shakers suffers at low reference frequencies because the efficiency of the coil in transforming the varying electromagnetic field into sinusoidal motion of the armature is poor at low excitation frequencies. Because helicopter vibration problems commonly occur at low frequencies, there remains a need in the art to provide a transducer tester that verifies the accuracy of a transducer at such low frequencies.